Oxygen welding and incorporating a novel gas separation system

ABSTRACT

An oxygen welding apparatus capable of generating its oxygen needs without an external oxygen source from a self-contained solid state electrolytic cell which separates oxygen from the air. The cell employs a flexible, ductile ceramic composite as the solid electrolyte. The ductile ceramic composite electrolyte comprises a continuous, ordered, repeating, interconnected ductile metallic array substantially surrounded by and intimately integrated within a ceramic matrix. The cell is connected to a power supply so when current is passed through the cell, oxygen or nitrogen is separated from the air passing through the cell.

This application is a divisional of allowed application Ser. No.07/821,458, filed Jan. 15, 1992, which issued as U.S. Pat. No. 5,332,483on Jul. 26, 1994, which is a continuation of of application Ser. No.07/549,467, filed Jul. 6, 1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to the separation of the oxygen andnitrogen from air, and more particularly relates to gas separation meansemploying a novel solid state ceramic composite electrolyte.

Oxygen has a broad range of medical, industrial and experimental uses.Most of the oxygen generating apparatus provided by the prior art isvoluminous and heavy due to the use of high pressure gas cylinders asthe oxygen supply source.

In recent years, there have been attempts to provide compact andlightweight oxygen generating systems that can supply oxygen gas forextended periods. Japanese Utility Model Publication No. 26445/1980discloses an oxygen gas generating system adapted to catalyticallydecompose aqueous hydrogen peroxide using a manganese compound as theCatalyst. This system has several drawbacks. The decomposition reactionaqueous hydrogen peroxide and manganese dioxide proceeds at anexplosively high rate if the volume of hydrogen periodixe is notcarefully controlled. If the volume and rate of the hydrogen peroxidereservoir is decreased to make the unit portable, the hydrogen peroxideis rapidly consumed and the reservoir must be replaced frequently. Forboth reasons, this is not a practical approach.

Japanese Patent Publication No. 42115/1977 employs a platinum catalystcapably of decomposing aqueous hydrogen peroxide at a highconcentration. This system is also unsatisfactory, both because itrequires a reservoir of hydrogen peroxide which must be periodicallyreplaced, and because of the expense and nature of the precious metalcatalyst. One problem with this approach is that the usual pore size ofthe alumina or silica gel catalyst support is too small to permitpenetration of the hydrogen peroxide. A major drawback is that theexpensive catalyst has a limited life. A further drawback is the precisetemperature control required.

One attempt to address the problems with hydrogen peroxide based oxygengenerating systems is disclosed in Japanese Patent Publication No.49843/1981 in which a system is provided for controlling the flow rateof hydrogen peroxide by valve adjustment using a link mechanism tocontrol the supply of aqueous hydrogen peroxide depending upon thepressure of the generated oxygen gas. However, the proposed system forconverting the gas pressure into mechanical displacement andtransmitting the displacement by means of the link has the drawback ofbeing unable to rapidly respond to the change in the reaction rate withresulting failures due to corrosion and abrasion in the actuatingsystem.

U.S. Pat. No. 4,792,435 discloses a system for produceing oxygen bycatalytic decomposition of aqueous hydrogen peroxide employing aplatinum group catalyst carried on a highly porous sintered ceramicsupport of large pore size. This system again suffers from the drawbackof requiring a hydrogen peroxide reservoir which must be periodicallyrecharged or replaced.

U.S. Pat. No. 4,784,765 provides an aquarium oxygen generator comprisinga container inverted into the apex or a ceramic cone-shaped ceramicstructure resting on the floor of an aquarium. Hydrogen peroxidesolution (15%) in the containiner is decomposed to form oxygen and waterin the presence of a catalyst pellet of finely divided silver admixedwith clay. Hydrogen peroxide seeps into the cone, and in the absence ofthe catalyst, reacts with organic material in the water to produceoxygen which bubbles through an aperture in the side of the cone-shapedstructure into the main body of water in the aquarium. While this systemmay be satisfactory for a small scale aquariium, it suffers from thedrawback of requiring a hydrogen peroxide reservoir and is not suitablefor medical, industrial and experimental uses.

The present invention solves the problems of the prior art and providesa system which generates oxygen or nitrogen from air, can be scaled upor down in size depending upon use, does not require consumables such ashydrogen peroxide or catalysts which must be replaced, and which isefficient and cost effective. The system of the present invention isbased on a novel, flexible and mechanically rugged, thin, solid stateelectrolyte ceramic composite.

Ceramics generally possess a number of desirable properties, includingmarkedly high resistance to abrasion, heat and corrosion compared tometallic materials. Certain ceramics, such as stabilized bismuth solidoxides, stabilized ceria solid oxides and zirconia solid oxides areionically conductive materials suitable for use as solid electrolytes.However, due to extreme brittleness, their application has been limiteddespite their other excellent properties.

A number of attempts have been made to increase toughness of ceramicmaterials by compounding them with another material including ceramic ormetal whiskers such as silicon carbide whiskers. Composites with ceramicmatrices and ductile metal inclusions such as those produced by LanxideCorporation show high fracture toughness when compared to ordinaryceramic materials. See for example U.S. Pat. Nos. 4,824,622; 4,847,220;4,822,759; 4,820,461; and related 4,871,008. These composites are achaotic, generally discontinuous, random metal dispersion in a ceramiccomposite body. They are prepared by a slow controlled oxidation ofmolten aluminum to alumina oxide, leaving behind approximately 5% of theparent metal. See also C. A. Anderson et al., Ceram. Eng. Sci. Proc., 9[7-8] pp. 621-626 (1988); and M. S. Newkirk et al., Ceram. Eng. Sci.Proc., 8 [7-8] pp 879-885 (1987).

P. Ducheyne et al., J. Materials Science 17(1982) 595-606 discloses abioglass composite produced by immersing premade porous fiber skeletonsinto molten bioglass to prepare metal fiber reinforced bioglass. Theseporous fiber skeletons produce random, chaotic, disordered supportmatrices and the process is applicable only to bioglasses.

U.S. Pat. No. 4,764,488 discloses a high toughness ceramic composite ofthe fiber-reinforced type wherein metal fibers having the shape oftriangular waves forming bent portions alternating on the opposite sideswith an angle O of the bent portions in a range between 60° and 165° anda d/H ration of between 0.025 and 0.6. While the discrete, discontinousfibers, unidirectionally anchored fiber reinforcement employed in the488 patent improve the toughness of the ceramic, this technique does notsolve the problem of crack propagation and ultimate failure.

U.S. Pat. No. 4,776,886 discloses a whisker-reinforced ceramic matrixcomposite comprising a principal crystal phase selected from the groupconsisting of anorthite, barium-stuffed cordierite and mixedcordierite/anthorite prepared by extrusion of ceramic batches comprisingan extrusion vehicle and a solid component comprising essentiallyinorganic whiskers and powdered glass.

The novel composite employed in the practice of this invention ismechanically tough. When subjected to intentionally severe mechanicalstress, such as bending a sheet in half and restraightening it, thecrack that resulted was limited to the stress or fold line.

This tough, ductile solid electrolyte composite permits the constructionof a gas separation device in which the only moving part is the airintake fan, and which does not require consumables such as hydrogenperoxide or catalysts requiring constant replenishment.

SUMMARY OF THE DISCLOSURE

The present invention provides a solid state device for separatingoxygen or nitrogen from air comprising a solid state electrochemicalcell wherein the solid electrolyte is a ductile composite comprising acontinuous, ordered, repeating ductile metallic array surrounded by andsupporting an ionically conductive ceramic matrix. The preferred form ofthe solid electrolyte is planar structure which can be fabricated intovarious configurations such as tubes, arcuate sections, corrugatedstructures or flat plates. Means are provided for connecting the cell toa power supply whereby when current is passed through the cell, oxygenor nitrogen is separated from air passing through the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an perspective view of one preferred oxygen generatorembodiment in accordance with the present invention with portions cutaway for clarity and understanding.

FIG. 2 is a fragmentary, cut-away elevation view showing greater detailof an electrolytic cell stack within the oxygen generator assembly.

FIG. 3 is an enlarged elevation side view of a ductile ceramicmultilayered single electrolytic cell unit employed in the oxygengenerator assembly of FIG. 1.

FIG. 4 is a 100× photomicrograph of the preferred embodiment of an openductile array for the solid electrolyte ceramic composite employed inthe practice of this invention.

FIG. 5 is a 60× SEM of a preferred embodiment of solid electrolytecomposite having a repeating pattern of the underlying diamond structurefrom the ductile array of FIG. 2.

FIG. 6 is a 50× optical photomicrograph of a solid electrolyte compositematerial employed in the practice of this invention.

FIG. 7 is a 6000× SEM photomicrograph of a section of a preferredembodiment of a solid electrolyte employed in this invention.

FIG. 8 is a photomicrograph of a section of the solid electrolyte ofFIG. 5 after it had been repeatedly bent 180° (in half) and straightenedto determine the effect of intentionally excessive mechanical abuse.

FIG. 9 is a graph of conductivity vs temperature of a solid electrolytecomposite using a 15 mole percent baria solution in bismuth oxide.

FIG. 10 is a graph of conductivity vs. temperature of a 20 mole percentsolution of baria in bismuth oxide.

FIG. 11 is a graph of voltage vs. current of the electrolyte of FIG. 9.

FIG. 12 is a graph of voltage vs. current of the electroyte of FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, depicting one preferred embodiment, oxygengenerator 10 comprises oxygen separator module 11. Oxygen separatormodule 11 comprises one or more individual solid electrolyte compositecells 30 (FIG. 3), placed in multiple plate stacked form 12, as shown inthis view. In this preferred embodiment, the plurality of solidelectrolyte cells are placed in modular stack array in which theindividual solid electrolyte cells are electrically connected in series.Bipolar stack housing 13 carries electrode connector unit 14. Powercontrol unit 16 may be disposed between power source 17 and the oxygenseparator module by current conductors 15. The oxygen separator unit mayeither be battery powered or plugged into a central remote generationsource.

In operation, air is drawn into intake unit 18 by air collection fan 19.The air is heated in heat exchanger 20 and travels into oxygen separatormodule 11, via preheated air intake line 25, where it is separated intooxygen and oxygen depleted air. The oxygen is drawn from oxygenseparation module 11 via oxygen conduit 21. Oxygen depleted air exitsmodule 11 through conduit 22. The hot air and oxygen are passed throughthe heat exchanger, and the resulting cooled oxygen product exitsthrough tube 23 while the cooled depleted air is released through vent24.

Referring to FIG. 2, the interior detail of the electrolyte plates andplenum chambers within electrolyte stack housing 13 is shown. Plates 30,shown in parallel sheet arrangement, are electrically connected inseries to each another by connectors 34. While these are depicted asseparate elements in this view, fabrication may be simplified by havingthe plates formed in block I cross-sectional form so that each contactsthose adjacent, with the end plate then having a block C cross section.Plenum chamber walls 31 are positioned on each side of all electrolyteplates in horizontal relationship so as to form a continuous stackwithin the housing. The plates are energized by the application ofcurrent.

Chamber walls 31 together with plates 30 form air plenum chamber 33 andoxygen plenum chamber 32. The chambers are continuous through housing 13and are gas isolated from one another. Spanning the chambers are currentpickups 35 formed of electronically conductive material such as metalstructures fabricated in the form of wool, expanded metal pieces, posts,rods, channels, ribbons or mesh which serve to pass current across thechambers but do not unduly impede gas flow. In operation, preheated airenters the top of the stack into chamber 32 at a temperature generallyslightly above 600° C. Oxygen within the air is transported in the formof oxide ions through the solid electrolyte plate and into the opposingoxygen plenum chamber 33. All air plenum chamber segments in the housingare gas parallel while all oxygen plenum chamger segments are likewisein gas parallel to one another and gas isolated from the air plenum.During passage of gases through the stack, application of a currentcauses a temperature increase, stabilizing typically at a temperature offrom 600° to 700° C.

Oxygen which has been transported through each electrolyte plate 30 asthe oxide ion collect in the oxygen plenum chamber on the opposing sidesof the plates. Pressure build up from the transport of oxygen into theplenum chamber units causes oxygen flow to commence. The oxygen depletedair and the oxygen gas travel separately in gas parallel through themodule and are conducted into the heat exchanger 20 through conduits 21and 22 where cold air collected through intake unit 18 (FIG. 1) isheated by the depleted air which is then vented, and by the oxygenproduct gas which is cooled in the heat exchanger and collected.

During normal operation, the system usually requires no supplementalheat addition but runs at steady temperatures. An auxiliary heater,preferably located in the heat exchanger, may be required during startup until steady operating temperatures are achieved.

The plenum walls are formed of material that is electrically conductiveand will withstand exposure to heated oxygen without being undulyoxidized. Preferred materials include 300 series stainless steel, 400series stainless steel, Incolloy 800 HT, super alloys including Inconel600 or 601 and Haynes 214.

In spacing the walls and electrolyte plates in stacked arrangement, asdepicted, it is advantageous to have the height of the air plenumchamber units greater than that of the oxygen plenum chamber. This is toaccommodate the larger volume of air passing through the module,compared to the small volume of oxygen being withdrawn. A ratio of 2:1,air plenum to oxygen plenum has been found to be quite suitable.Increasing the height of the chambers has the advantage of reducingpressure drop caused by the packing, but it has the disadvantage ofincreasing the overall size of the stack.

The current pickups 35 within the plenum chambers contact the electrodesand the bipolar plates to avoid short circuits within the stack.Electricity passes throught the plenums perpendicular to the gas flowwhich is horizontal. The pickups are suitably composed of the samematerials as that used to form the plenum walls 31. While goodelectrical contact is needed to avoid undue voltage drop, the moreporous the pickups are, or the less resistance they offer to gas flow,the better they operate by lessening restriction of gas flow andavoiding excessive pressure drop through the chambers. FIG. 3 depicts inlarge scale the layers forming the cell 30 which is the essentialcomponent of this invention. Solid electrolyte ductile ceramic composite40 is the center layer of the cell. It is preferably sandwiched betweenmixed conductor layers 41 formed of solid electrolyte doped withmulti-valent ions which make the layers both ionically andelectronically conductive. The mixed conductive layers 41 are coatedwith electrically conductive gas porous electrode thin layers 42 whichform an anode and a cathode on the surface of the cell.

The mixed conductor component 41 is formed of a solid electrolyte suchas bismuth oxide or zirconia. The multi-valent doping materials whichare suitable include praseodymium, terbium, cesium, iron and chromium.

The electrically conductive electrode surfaces on each side of the cellare preferably silver, silver alloys or conductive oxides such asperovskites.

The ductile, tough solid electrolyte ceramic composite employed in thepractice of the present invention comprises a regular, ordered,continuous, repeating array of ductile intersupported or interconnected,metallic fibers in intimate contact with the ceramic matrix so as to besubstantially surrounded or embedded within it and supporting thematrix. The ceramic employed in the practice of this invention is ahighly ionically conductive material. Preferred ceramic phase materialsin which the fibers are embedded are solid oxide electrolytes based onsolid solutions of bismuth oxide and a second component selected from ametal oxide wherein the metal ion has a valence of +2, +3, +5, or +6.The purpose of the stabilizing agent is to hold the bismuth oxide in thepreferred crystal lattice symmetry in a temperature range at which itwould otherwise convert to a less conductive, or non-conductivepolymorph. In general, the optimum lattice symmetry is face centeredcubic. However, the alkaline earths form rhombohedral phases withbismuth oxide and these phases are also extremely conductive.

Table I is a partial list of bismuth solid oxide electrolytes which maybe employed in the practice of this invention.

                                      TABLE I                                     __________________________________________________________________________    Bismuth Solid Oxide Electrolytes                                                                 Ω.sup.-1 cm.sup.-1 at                                Doped-Oxide                                                                           Electrolyte                                                                              450° C. (× 10.sup.3)                                                     E/E° @ T °C.                                                            Reference                                  __________________________________________________________________________    Niobium (Bi.sub.2 O.sub.3).sub.0.85 (Nb.sub.2 O.sub.5).sub.0.15                                  3.5     0.98 @ 600                                                                            JES, 124, 1563 (1977)                      Vanadium                                                                              (Bi.sub.2 O.sub.3).sub.0.875 (V.sub.2 O.sub.5).sub.0.125                                 1.8     0.97 @ 600                                                                            JES, 124, 1563 (1977)                      Yttrium (Bi.sub.2 O.sub.3).sub.0.75 (Y.sub.2 O.sub.5).sub.0.125                                  3.2     0.98 @ 550                                                                            JAE, 5, 187 (1975)                         Yttrium (Bi.sub.2 O.sub.3).sub.0.80 (Y.sub.2 O.sub.5).sub.0.20                                   12              MRB, 21, 1215 (1986)                       Tungsten                                                                              (Bi.sub.2 O.sub.3).sub.0.78 (WO.sub.3).sub.0.22                                          6.0     0.99 @ 550                                                                            JAE, 3, 65 (1973)                          Strontium                                                                             (Bi.sub.2 O.sub.3).sub.0.8 (SrO).sub.0.2                                                 2.6     0.97 @ 550                                                                            JAE, 2, 97 (1972)                          Cadmium (Bi.sub.2 O.sub.3).sub.0.6 (CdO).sub.0.4                                                 25      0.0  @ 500                                                                            JAE, 2, 97 (1972)                          Lanthanum                                                                             (Bi.sub.2 O.sub.3).sub.0.9 (La.sub.2 O.sub.3).sub.0.1                                    5.0     0.92 @ 550                                                                            JAE, 2, 97 (1972)                          Lanthanum                                                                             (Bi.sub.2 O.sub.3).sub.0.85 (La.sub.2 O.sub.3).sub.0.15                                  8.1     0.96 @ 500                                                                            JSSC, 39, 173 (1981)                       Samarium                                                                              (Bi.sub.2 O.sub.3).sub.0.9 (Sm.sub.2 O.sub.3).sub.0.1                                    5.1     1.0  @ 500                                                                            JSSC, 39, 173 (1981)                       Neodymium                                                                             (Bi.sub.2 O.sub.3).sub.0.9 (Nd.sub.3 O.sub.3).sub.0.1                                    6.9     0.98 @ 500                                                                            JSSC, 39, 173 (1981)                       Erbium  (Bi.sub.2 O.sub.3).sub.0.75 (Er.sub.2 O.sub.3).sub.0.25                                  3.9     1.02 @ 500                                                                            JSSC, 39, 173 (1981)                       Erbium  (Bi.sub.2 O.sub.3).sub.0.8 (Er.sub.2 O.sub.3).sub.0.2                                    7.7     0.98 @ 500                                                                            JAE, 10, 81 (1980)                         Molybdenum                                                                            (Bi.sub.2 O.sub.3).sub.0.6 (MoO.sub.3).sub.0.4                                           1.1     0.97 @ 600                                                                            JAE, 7, 31 (1977)                          Codolinium                                                                            (Bi.sub.2 O.sub.3).sub.0.9 (Gd.sub.2 O.sub.3).sub.0.4                                    3.5     1.0  @ 600                                                                            JAE, 5, 197 (1975)                         Barium  (Bi.sub.2 O.sub.3).sub.0.8 (BaO).sub.0.2                                                 5.2     0.97 @ 450                                                                            JSSC, 15, 317 (1976)                       Barium  (Bi.sub.2 O.sub.3).sub.0.84 (BaO0.sub.0.16                                               15              JMS, 20, 3125 (1985)                       Praeseodymium                                                                         (Bi.sub.2 O.sub.3).sub.0. 8(Pr.sub.2 O.sub.11/3).sub.0.2                                 1.9     0.98 @ 500                                                                            JAE, 12, 235 (1982)                        Praeseodymium                                                                         (Bi.sub.2 O.sub.3).sub.0.9 (Pr.sub.2 O.sub.11/3).sub.0.1                                 4.0             JAE, 12, 235 (1982)                        Terbium (Bi.sub.2 O.sub.3).sub.0.8 (Tb.sub.2 O.sub.3.5).sub.0.2                                   0.61   0.96 @ 500                                                                            JAE, 15, 447 (1985)                        Terbium (Bi.sub.2 O.sub.3).sub.0.9 (Tb.sub.2 O.sub.3.5).sub.0.1                                  4.1     0.87 @ 500                                                                            JAE, 15, 447 (1985)                        __________________________________________________________________________

The first column in Table 1 is the stabilizing agent which may be analkaline earth, a lanthanide or a transition metal. The second column isthe composition of the solid solution which is reported to exhibit thehighest conductivity for a given pairing of metal ions. The third columnlists the reported ionic conductivity at the somewhat arbitrarytemperature of 450° C. The barium stabilized phase, with a bariumcontent of 15-20 mole percent of stabilizing oxide formula as written isone of the most conductive.

The fourth column in this table is the transference number for the oxideconduction at the stated temperature. The transference number for oxideconduction is the fraction of the current which is carried by oxideions, instead of by semiconducting or metallic mechanism. In general, auseful solid electrolyte must have a transference number in excess of95% , meaning that approximately 5% or less of the current is carried bynon-electrolytic mechanisms. Note that all of the compositions exceptfor cadmium and terbium meet this fundamental criterion. The zerotransference number of the cadmium phase indicates that it is a pureelectronic (not electrolytic) conducter.

The fifth column lists literature references. In Column 5, JES refers toJournal of the Electrochemical Society, JAE refers to Journal of AppliedElectrochemistry, MRB refers to Materials Research Bulletin, JSSC refersto Journal of Solid State Chemistry, and JMS refers to Journal ofMaterials Science.

It is presently preferred to employ the bismuth baria rhombohedralsystem wherein the barium stabilized phase has a barium content of 15-25mole percent of stabilizing oxide formula, preferably 15-20 molepercent, and most preferably 20 mole percent.

Ceria stabilized with a metal oxide wherein the metal ion has a valenceof +2 or +3 may also be used in the practice of this intention as theceramic phase of the solid electrolyte composite. Representativestabilizing agents are oxides of yttrium, scandium, gadolinium and otherrare earth and alkaline earth matals.

As best shown in FIG. 4, a preferred embodiment of the ductile componentof the solid electrolyte composite employed in the practice of thisinvention is an intersupported, planar array of metallic ligamentsforming a repeating diamond pattern. The line of sign openess of thisarray is about 65-70%. Ceramic volume fraction of the solid electrolyteceramic composite composition is from 10% to 95%. The preferred ceramicvolume fraction of the final ceramic composite is about 90%.

The preferred material for the ordered, ductile array is a single layerof an open mesh metal structure. Especially preferred are expanded metalfoils such as Haynes 214 expanded metal foil. Especially preferred is anexpanded metal foil produced in accordance with this invention from asolid sheet of Inconel 600 foil with an original thickness of 0.003".

The solid electrolyte composite employed in the practice of thisinvention is a thin sheet-like structure having a thickness of 0.01 inchor less. It is preferred that the composite have a thickness of 0.003inch or less. While one of the retirements of structures in which thecomposite is used, including the oxygen generator module, is that thecomponents including the electrolyte composite be of sufficientmechanical strength to withstand stresses to which they will be exposed,this will normally dictate the thickness required. If very large sheetsnot supported by the current pickups are employed, for example in theform of posts, mechanical strength retirements will increase and greaterthickness will be required.

Generally speaking, the shape of the composite body is irrelevant to itsoperation. It may be square, rectangular, circular, pleated corrugated,and the like. For best results it is preferred that the composite bodyemployed in the electrolytic cell, which forms the essential element ofthe oxygen generator of this invention, is at least 4 inches on a side,preferably 6 inches or more in diameter if round to provide anequivalent surface area. Size will depend upon the end application.Portable oxygen generators for medical or other personal use, such asfor firefighters, would use relatively small cells. For large,industrial applications, composite bodies having dimensions of 1 to 2meters or more per side may be employed.

As shown in FIG. 5, in the solid electrolyte composite formulated inaccordance with a preferred embodiment of the present invention, thereis a repeating pattern of the underlying diamond structure of theductile array. The EDX analysis of the interface between metal and solidelectrolyte ceramic showed a "metal oxide" with the composition CrNi₂O_(x). The EDX analysis of the metal ligaments was consistent with thepublished values of Inconel 600 while the bulk ceramic phase wasconsistent within the precision of the EDX unit with the intended solidsolution of bismuth and barium oxides as shown in FIG. 8.

FIG. 6 is a 50× optical photomicrograph of a composite of thisinvention. The "diamonds" of ceramic oxide solid electrolyte with theinterconnecting lines of metallic ligaments can be seen. Uponbacklighting, the composite clearly showed its form with a yellow-orangetransmitted light interrupted in a precise regular repeating array ofopaque (metallic) lines. The optically transmitting regions were thediamond shaped ceramic filled subsections.

FIG. 7 is a 6000× SEM photomicrograph of a section of the solidelectrolyte prepared in accordance with Example 2. The white occlusionsare unreacted nearly pure bismuth oxide.

The solid electrolyte composite of this invention was found to be quiteflexible, capable of flexing out of plane by as much as 0.25 inch ormore with finger tip pressure on a sample of about two inches in length.A sample was repeatedly bent to 180° (folded in half) and straigthenedto examine the effect of such mechanical abuse.

FIG. 8 is a photomicrograph of such a sample. As can be seen, despitethe extreme mechanical abuse, a resulting crack only formed along theline of maximum stress or fold line. However, there was no crackpropagating away from the fold line, and the crack that did appear didnot even extend within a given, unsupported ceramic diamond area. Thesame ceramic composition, outside of the composite structure, wouldshatter. Prior art composite structures would not withstand such abuse.

The cell of FIG. 3 was tested under the following conditions. Cell 30comprised a symmetric "sandwich" with the ionically conductive solidelectrolyte composite as the center layer 40. On either side of thesolid electrolyte center layer are mixed conductive layers 41 which arecoated with electronically conductive porous metallic layers 42. WovenInconel 600 cloth was coated with commercial silver based paste to actas current collector and to allow for the passage of gases inpassageways 32 and 33 which serve as the oxygen and air plenumsrespectively. The plenum walls were 1/8 inch thick sheet of Haynes 214alloy to serve as current pickups. A single fine platinum wire wasplaced on the cathodic side of the cell composite in contact with theelectrolyte but not in direct electrical contact with the electrodes orthe metal current collectors of pickup plates. There was static air onboth sides of the sample at the beginning of each test. The cathode sidebecame depleted in oxygen while the anodic side oxygen partial pressureincreased. The test cell was placed in an electric Nichrome wound mufflefurnace and the temperature was raised to the appropriate level. Athermocouple, independent of the furnace thermocouple, was placed indirect contact with the test array at all times. Voltage was appliedwith a small 15 amp DC power supply from Darrah Electronics. Thevoltages, currents, and DC resistivities were read using digitalmultimeters. FIGS. 9 and 10 are graphs plotting data points measuredwith the above apparatus.

FIGS. 9 and 10 depict show the DC conductivity versus inversetemperature behavior of composites of this invention having 15 and 20mole percent of BaO in the ceramic composition phase respectively. Solidelectrolytes should exhibit a linear relationship between the log of theconductivity (resistivity) and the inverse Kelvin temperature.

FIG. 9 depicts data taken on a solid electrolyte composite of thisinvention prepared from a 15 mole percent baria (BaO) solution inbismuth oxide (Bi₂ O₃). The data were taken over a temperature range ofthree hundred degrees Kelvin and includes the operating temperaturerange of the oxygen generator of this invention. The slope of the datais about twenty two kilocalories/mole which is consistent with thepublished literature values for the bulk ceramic, The least squarescorrelation coefficient for seven data points over the three hundreddegree range is >0.98.

FIG. 10 is a related data curve taken on a composite produced utilizinga 20 mole percent solution of baria in bismuth oxide. The 15% solidelectrolyte showed a small but finite level of the monoclinic phase (byXRD), while the 20% baria solution showed only the pure rhobohedralphase at the precision level of XRD. The 20% curve (FIG. 10) exhibited asomewhat steeper conducitivy vs. temperature slope than the 15% bariaceramic composite. This results in an energy of activation for the 20%baria material of about 26 KC/mole, slightly higher that the 15%material. At the highest temperature measured, the curve for the 20%baria material may be exhibiting the change in slope as reported bySuzuki [JMS, 20, 1985, 3125] and others for the bulk ceramic.

FIGS. 11 and 12 depict the voltage versus current behavior of theseelectrolytic ceramic composites. The vertical axis is the logarithm ofthe current in amperes or in current density. The horizontal axis is thevoltage between the cathodic electrode and the platinum referenceelectrode less the open circuit potential of 14-15 mV depending ontemperature.

Generally speaking, the high melting temperature electrolyte compositesemployed in the practice of this invention are prepared by preparing aslurry of fine metal oxide powder having a particle size under 1 micronto form a doughy slurry, adding an organic binder, preferably under0.25% of a binder such as polyvinylalcohol, pouring or otherwisedistributing the ceramic phase solution over the ductile support arrayto be embedded therein, firing in a reducing or inert atmosphere toapproximately 1000° to 1400° C., preferably 1200° to 1350° C. and mostpreferably 1300° to 1350° C. for from 1 to 24 hours, cooling andrepeating the cycle until there is >90% density in the ceramic.

It is especially preferred to anneal the composite under a directedenergy source such as a carbon dioxide laser or electron beam. In thisway, the ceramic can be heated above its melting point, permitting it toflow evenly around the ductile array, while the metal remains under itsmelting point. Beam rastering rates of approximately 1 inch/sec workespecially well.

The following examples further illustrate the invention.

EXAMPLE 1

A slurry of a molten hydrate melt of (CeNO₃)₃ 6H₂ O, Gd(NO₃)₃ H₂ O andCe₀.8 Gd₂ O₁.9 was applied to Inconel 600 mesh (60 mesh) stainlesssteel, suspended in a furnace with 0.008 inch Inconel wire and fired toapproximately 650° C. After cooling, the composite was laser annealedusing a CO₂ laser having a 10.6 wavelength, 600 W, 3/8 inch by 0.005inch. The ceramic melted, flowed and refroze without melting the metalsuport matrix. The annealing was done under flowing argon. A sample ofcomposite was held on a computer controlled table and rastered under theCO₂ laser beam at a rate of 1 inch/sec. Very slow rates vaporized thesample and faster rates insufficiently melted the ceramic.

EXAMPLE 2

A bismuth baria solid electrolyte composite wherein the ceramic phasecontains 20 mole percent baria was prepared as follows.

Bismuth oxide (Bi₂ O₃, 150 g) was mixed with Ba(NO₃) (11.7 g) and Bi₂ O₃(21 g) [Bi₂ O₃)₀.72 (BaO)₀.28 and poured into an alumina tray containing340 g of 16% BaO. Upon melting, the final composition is (Bi₂ O₃)₀.80(BaO)₀.20. The mixture was well stirred and heated to a temperature of900° C. for about 11/2 hours, then cooled to 860° C. A ductile array ofInconel 600 expanded metal foil having a line-of-sight openess of about70% and forming a regular, structured, repeating diamond pattern waspreoxidized by heating to approximately 700° C. for about 11/2 hours inair. The preoxidized metal support or ductile array way dipped into theliquid ceramic phase to coat the ductile array with the ceramic phase,cooled and annealed at a temperature of 780° C.

EXAMPLE 3

On a 90 mm diameter Buchner funnel there was placed a sheet of Whatman541 filter paper. The funnel assembly was covered with a 1/8 inch thickneoprene gasket sheet having a 90 mm diameter. The neoprene gasket had arectangular hole somewhat smaller than the composite sample. A secondpiece or rubber neoprene was placed in the hole on top of the Whatman541 sheet to physically support the composite without sealing. Thesystem, without a composite sheet, was wetted with ethanol and allowedto set. The composite prepared above was placed over the neoprene holeand sealed in place with a neoprene gasket.

A thin layer of the above bismuth baria slurry was painted on andallowed to dry for about 5 minutes. The sample was dried at 60° C.,recoated on the opposite side and dried again. The composite was thenfired at 700° C. in air for about 20 hours. A second coat of the bismuthbaria slurry was applied and the composite fired at 700° C. for 15 hoursin air. A third coat was applied and fired at 700° C. in air for 1 week.

EXAMPLE 4

A solid electrolyte composite was prepared following the method ofExample 3 with the following modifications. The surface of the compositewas painted with a slurry of BiBaOx and 20% polyethyleneimine (50%aqueous) under suction in a Buchner funnel. The composite was dried at125° C., the opposite side coated and dried as above. The composite wasplaced under a weight and fired in air at 680° C. for 20 hours, cooledto room temperature and both sides were painted with 1% aqueouspolyethylenimine and partially dried. Both faces of the composite werepainted with silver palladium paint and dried at 150°-200° C. A secondcoat of silver palladium paint was applied and the composite fired at350° C. for 1 hour in air and cooled to room temperature. The compositecell was placed in a furnace under a weight and the temperature raisedto 700° C., held for 10 minutes, reduced to about 550° C., and thenheated in air at 700° C. for 14 hours.

EXAMPLE 5

A cell body was constructed from a section of standard schedule 40 threeinch SS316 pipe with external standard threads cut in one end. A planardisc of Inconel 600 expanded metal foil was tack welded on the endwithout the threads. The expanded foil disk was five inches in diameter.The outer one inch was cut radially into tabs approximately 0.5 inchescentered over the unthreaded end of the pipe section and the tabs werebent down over the exterior sides of the pipe. The tabs were tacked inplace with five welds of approximately 1/16 inch in diameter arranged ina three/two pattern with the three at the far (wider) end of the tab.Additional welds were tacked every two or three mm along the upper rimof the pipe at approximately 1/8 in intervals.

Following the method of Example 2, a ceramic phase of bismuth bariaoxide solid solution was prepared and melted in an Inconel 600 deepdrawn crucible. The crucible was placed in an oven and heated to 925° C.for about 30 minutes. The crucible was periodically swirled to insure &chemically homogeneous melt and the temperature was reduced toapproximately 850° C.

During the 30 minute thermal hold of the above procedure, the cell bodywas placed in the oven along side the crucible to heat it to thetemperature of the melt. This is important because if cold metal isdipped into the melt, the relatively large thermal mass of metal coolsthe melt to below the solidus temperature which results in the meltfreezing and the pipe section fusing to the frozen ceramic mass. Itgenerally requires a minimum of 30 minutes to raise the metal to theappropriate temperature.

The furnace door was opened and the metal section grasped with longtongs and dipped into the open-topped crucible containing the melt. Thecell was promptly removed from the furnace and placed on a concretesurface to cool. The composite top of the cell cooled to roomtemperature within seconds, although the pipe section took severalminutes to cool. The cell unit was examined for pinholes and none werefound.

Silver palladium paste was applied to the interior and exterior surfacesof the composite. The paste was dried at 110° C. for 20 minutes andfired at 700° C. for an additional thirty minutes. Several coats wereapplied using this procedure.

The room temperature electrical resistance between the interior coat(the anode, oxygen evolution site) and the exterior coat (the cathode,the oxygen dissolution rate) was >30,000,000 ohms, the limit or theDMMs. This indicates the electrodes were not short-circuited. Electricalresistance between any two points on a given electrode at roomtemperature was about 0.2 ohms or less.

The exterior electrode was approximately two inches in diameter and didnot make direct electrical contact with the pipe. The silver paste ofthe interior electrode was intentionally spread onto the interior wallsof the pipe, making an electrical connection between the housing and theinterior electrode. There was no measurable room temperature electricalconductivity between the pipe and the exterior electrode.

A SS316 reducing union piece was then threaded onto the open end of thepipe section using high temperature thread sealant. The small end of thereducing union was connected to 1/8 inch stainless steel tubing using aSS 316 swagelock adapter. The 1/8" tubing extended out of a hole in thetop of the furnace. About 18 inch of small diameter tubing extended outof the furnace. The "cold" end of the SS tube was connected to asection, about 3 feet long, of standard 1/8 inch i.d. Tygon tubing. Thiswas the gas circuit.

When electrical power was applied to the cell, at temperatures of about650° C., oxygen was produced at the anode. This was detected byimmersing the end of the tygon tubing in a small dish of water andseeing a steady stream of bubbles. In the absense of electrical power,the flow of oxygen gas (bubbles) ceased.

The oxygen generator system of the present invention has a wide varietyof applications. It can be fabricated into a light-weight, portable unitfor medical use or use by firefighters and other individuals who areworking in situations where an independent oxygen supply is needed. Itcan be fabricated into large industrial units to supply oxygenrequirements in industrial processes. It can be used as an oxygen sourcefor operations such as welding. It is versatile, its only moving part isa fan, it does not require replacement of consumables such as hydrogenperoxide or catalyst, and can continuously generate a supply of oxygenfor prolonged periods of time. It can be fabricated into any desiredsize or shape to meet the desired application.

The invention claimed is:
 1. An oxygen welding unit which generates itsoxygen needs without an external oxygen source, said welding unitcomprising: a housing, air ingress means associated within said housing,an oxygen generating module disposed within said housing, air intakemeans, oxygen egress means and spent air egress means, said oxygengenerating module comprising a first solid electrolyte composite cell,said cell having a center sheet of a flexible, ductile solid electrolytecomposite, said flexible, ductile, solid electrolyte compositecomprising a regular, ordered, continuous, repeating array of ductileinterconnected metallic ligaments in intimate contact with andsubstantially surrounding by an ionically conductive ceramic matrix;said flexible ductile composite sheet having an upper surface and anopposing lower surface, a layer of electronically conductive, gas porousmaterial coating each surface of said ductile composite sheet where acathode is formed on one of said surfaces and an anode is formed on theopposing surface of said cell; means for supplying power to said oxygengenerating module whereby when air is passed through said oxygengenerating module, the air is separated into oxygen and depleted air; anoxygen conveyor assembly having an entry open end, an exit port andfluid-tight side walls, said entry open end connected in fluid tightengagement with said oxygen generator module; means for conveying fuelto said oxygen generator module comprising an entry open end, an exitport and fluid-tight side walls, the entry end in fluid-tight attachmentto a fuel source, and the exit port mixedly attached to the exit port ofthe oxygen module; and means for regulating the oxygen gas and fueldelivered at the exit port when gas and fuel are delivered from theoxygen generator and the fuel supply reservoir whereby when the combinedoxygen and fuel stream is ignited, a welding flame is obtained.
 2. Anoxygen welding unit according to claim 1 which cooperates with anexternal fuel source.
 3. An oxygen welding unit according to claim 1additionally comprising a fuel reservoir.
 4. An oxygen welding unitaccording to claim 1 wherein said flexible ductile solid electrolytecomposite cell additionally comprises a layer of mixed ionic andelectronic conductive material coating each of said surfaces of thecenter sheet between said surfaces and said anode and cathode.
 5. Anoxygen welding unit of claim 1 wherein said ductile metallic array is anexpanded metal foil, woven metal, braided metal fibers or metal mesh.